The present invention relates generally to MR imaging and, more particularly, to a method and apparatus of slice selective magnetization preparation for moving table MRI. More specifically, the present invention relates to the timing and positioning of preparation RF pulses, e.g. inversion recovery pulses, in a pulse sequence for whole body axial imaging of a patient being translated through an imaging volume of an MR system by a moving table.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Moving table MRI is an MR imaging technique that allows for whole body imaging in a relatively short amount of time. Ideally, through continuous translation of a patient through an imaging volume, MR data can be acquired of the chest, abdomen, and pelvis in a single breath-hold. Furthermore, it is preferable for a moving table MR study to provide image quality, contrast, and resolution comparable to stationary or non-moving table studies. Stationary-table studies frequently implement contrast-preparation techniques such as inversion recovery (IR) and saturation recovery to enhance contrast in a reconstructed image. These imaging techniques utilize spatially selective or spatially and spectrally selective RF pulses at a fixed interval in time prior to application of imaging RF pulses and readout gradient pulses. These traditional imaging protocols, however, are not optimal for moving table MRI.
As suggested by its name, moving table MRI uses a table to translate a patient through an imaging volume during the imaging process. The table may incrementally or continuously move the patient through the imaging bore of the MR system. Unlike stationary-table imaging techniques, in moving table MRI, the patient is translated or moved during data acquisition. As such, if a preparatory RF pulse is applied at a moment in time to a particular slice or slab of the patient that is fixed relative to the imaging bore, the tissues subjected to the preparatory pulse will move in the direction of table motion over time. Therefore, a region or volume of interest that was marked for data acquisition may move out of the slice or slab and, as such, not present data for acquisition.
A standard IR pulse sequence 2 for obtaining T1-weighted images of a patient positioned on a stationary stable is illustrated graphically in FIG. 4. As is known, the magnetic moments of the spins in tissue are uniformly aligned upon placement of a patient in a uniform B0 field. This magnetization 3 is then inverted by the application of an RF inversion pulse 4, i.e. 180 degree flip angle, to an imaging volume. Coinciding with the application of the inversion recovery pulse is a slice select gradient 5 that selectively encodes the imaging volume. After the IR pulse 4 is applied, an inversion recovery time TI is observed that allows for the magnetization 3 to recover and decay in accordance with T1 and T2 characteristics of the tissue. The longer TI, the more recovery and decay in the magnetization. At the expiration of TI, another RF pulse 6 as well as slice selective gradient 7 is applied. This RF pulse 6 is generally referenced as an imaging pulse. RF pulse 6 is applied to drive the magnetization of the spins in the tissue back to the transverse plane. Because the RF pulse 6 is preferably applied at TI, those spins that had a zero magnetization are nulled. As illustrated, for T1-weighted imaging, an imaging module 8, i.e. phase encoding and frequency encoding gradients, is applied relatively immediately after RF pulse 6 to acquire MR data from the non-nulled magnetization.
When applied to moving table MRI it is clear that the spins to which the IR pulse was directed, will not only recover toward equilibrium longitudinal magnetization during TI but will move in the direction of table motion. Depending upon the slice/slab thickness set for the particular imaging session and the velocity of table translation, the spins may no longer be in the slice, slab, or volume of interest when TI expires and, as such, not detected during readout.
One possible solution is to adjust the timing of the imaging RF pulse relative to the preparation RF pulse so that the TI period is changed. However, the user selects the appropriate TI period typically as a function of the T1 values of the targeted tissue. Since a preparation RF pulse, such as an inversion RF pulse, causes magnetization to be driven below the transverse plane, the magnetization will progress toward positive magnetization from the inverse or negative magnetization. As such, adjusting TI will not achieve the effect sought with preparation RF pulses and, as such, affects image contrast as well as image intensity; both of which could negatively affect the diagnostic value of the reconstructed image.
Another proposed solution is to acquire imaging data during TI, i.e. as magnetization recovers. However, if data from the imaging volume is collected during the approach from inversion to fully recovered magnetization image contrast will change on a slice-by-slice basis. Variations in image contrast on a per slice basis also negatively affect the diagnostic value of the reconstructed image.
It would therefore be desirable to have a system and method capable of achieving slice selective magnetization preparation for moving table MRI.